1. Field of the Invention
The present invention relates to radiation sensitive mixtures (e.g., those particularly useful as positive-working resist compositions) containing the admixture of an alkali-soluble binder resin, a photoactive compound and an effective sensitivity enhancing amount of at least one polylactide compound all dissolved in a solvent. Furthermore, the present invention also relates to substrates coated with these radiation sensitive mixtures as well as the process of coating, imaging and developing these radiation sensitive mixtures on these substrates.
2. Brief Description of Prior Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. In these processes, a thin coating or film of a photoresist composition is generally first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The preferred method of applying this film is spin coating. By this method, much of the solvent in the photoresist formulation is removed by the spinning operation. The coated substrate is then baked to evaporate any remaining solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam, ion beam, and X-ray radiant energy are radiation types commonly used today in microlithographic processes.
After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate. In some processes, it is desirable to bake the imaged resist coating before this developing step. This intermediate step is sometimes called post-exposure bake or PEB.
There are two types of photoresist compositions--negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g., a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the nonexposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited but not exposed to the radiation. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g., the Wolff rearrangement reaction of the photoactive compound occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. The desired portion of the underlying substrate surface is uncovered where the photoresist was exposed to the radiation.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate are protected where the photoresist coating still remains and, thus, an etched pattern is created in the substrate material which corresponds to the photo-mask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
End users of photoresists are demanding photoresist formulations possess better lithographic properties for the fabrication of smaller micro-electronic circuits. The lithographic properties which are critical to positive-working photoresist end-users include the following: (1) good resolution capabilities in both the micron and submicron ranges without incomplete development in the exposed areas (i.e. scumming); (2) higher thermal image deformation temperatures (e.g., above 120.degree. C.); (3) relatively fast photospeeds; (4) good adhesion to substrate; (5) good developer dissolution rates; (6) good process latitude; (7) near to absolute vertical profiles (or good contrast) between exposed and unexposed photoresist areas after development; (8) good resistance to etching solutions and plasma etching techniques; (9) reduced tendency to form insoluble particulates; and (10) mask linearity.
Generally in the past, efforts to improve one of these lithographic properties have caused significant decreases in one or more of the other lithographic properties of the photoresist. Accordingly, there is a need for improved photoresist formulations which possess all of these desired properties without making significant tradeoffs. The present invention is believed to be an answer to that need.
For example, it is well known to add sensitivity enhancers (also known as photospeed enhancers or speed enhancers) to resist formulation to increase the solubility of the resist coating in both the exposed and unexposed areas when the speed of development is an overriding processing consideration. However, some degree of contrast may be sacrificed with the addition of such sensitivity enhancers, (e.g. in positive-working resists, while the exposed areas of the resist coating will be more quickly developed, the sensitivity enhancers will also cause a larger loss of the resist coating from the unexposed areas). Thus, if too much resist coating is removed from the unexposed areas of a positive-working resins, film defects such as pinholes may be introduced into the coating or subsequent plasma etching steps may cause unwanted breakthroughs in the unexposed areas. Accordingly, sensitivity enhancers should provide the desired increased speed of development without too much more film loss in the unexposed areas.
Numerous compounds have been proposed as sensitivity enhancers in resist compounds. See U.S. Pat. Nos. 3,661,582; 4,009,033; 4,036,644; 4,115,128; 4,275,139; 4,365,019; 4,650,745; and 4,738,915 for examples of known sensitivity enhancers. All of these U.S. Patents are incorporated herein by reference in their entities. While their known sensitivity enhancers may be suitable for some resist formulation or for some particular end uses, there is a need for new sensitivity enhancers which have better sensitivity enhancement without significant film loss in other resist formulations or in other end uses, or are suitable in a certain combination of resist formulations or a combination of end uses to which the previously known sensitivity enhancers are not suitable. The present invention is believed to be an answer to this need.
Separately, ethyl lactate [CH.sub.3 CH(OH)COO2C.sub.2 H.sub.5 ] is a known solvent for positive-working photoresists. In the presence of acidic substances, it forms polylactides [defined as ##STR1## wherein n is from 2 to 10] by the following reaction sequence: ##STR2##
It has been found that if positive-working resists are treated with cation exchange resin, a portion of the ethyl lactate solvent in the photoresist formulation may be converted into a polylactide. It has been found that polylactides act as speed enhancers or sensitivity enhancers in positive-working photoresist compositions.